1. Field of the Invention
The present invention relates to a process for the electrolysis of hydrogen chloride in aqueous solution, and more particularly, to reducing cell voltage at a given current density or enhancing current density in an electrolytic cell having thin film electrodes.
2. Description of the Related Art
Aqueous hydrogen chloride (HCl) or hydrochloric acid is a reaction byproduct of many manufacturing processes that use chlorine. For example, chlorine is used to manufacture polyvinylchloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a byproduct of these processes.
Because supply so exceeds demand, byproduct hydrogen chloride or hydrochloric acid often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl byproduct. A number of commercial processes have been developed to convert HCl into usable chlorine gas.
Currently, thermal catalytic oxidation processes exist for converting anhydrous HCl and aqueous HCl into chlorine. Commercial processes, known as the "Shell-Chlor", the "Kel-Chlor" and the "MT-Chlor" processes, are based on the Deacon reaction. This reaction was originally developed in the 1870's using a fluidized bed containing a copper chloride salt which acts as the catalyst. HCl reacts with oxygen to produce chlorine gas and water. Commercial improvements to the Deacon reaction have involved use of alternatives to the copper chloride catalyst; promoters to improve the rate of conversion and to reduce the energy input; and methods to reduce the corrosive effects on processing equipment by the harsh chemical reaction conditions. However, in general, thermal catalytic oxidation processes require complicated separations of the reaction components to achieve product purity. These processes also require expensive construction materials for the highly corrosive reaction systems, which operate at temperatures of 250.degree. C. and above.
Electrochemical processes exist for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution. The current electrochemical commercial process is known as the "Uhde" process. In this process, aqueous HCl of about 22 wt % is fed at 65-80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17 wt %, with the production of chlorine and hydrogen gases. A polymeric separator divides the two compartments. The process requires recycling of the dilute (17 wt %) HCl solution (generally by adsorbing gaseous, anhydrous HCl) to regenerate HCl solution of 22 wt % for feed to the cell. This means the process generally operates on anhydrous HCl feed even though the medium is aqueous. If HCl concentration becomes too low, a side reaction may occur whereby oxygen is generated from the water present in the system, which increases operating costs. Further, use of HCl as the supporting electrolyte in the Uhde system limits current densities at which the cells can perform to less than 500 amps/ft.sup.2 (0.54 amps/cm.sup.2). As HCl is converted to chlorine, the electrolyte becomes depleted of ions, increasing resistance to the flow of current and, potentially, causing side reactions to occur. Aqueous HCl that is present in the gap between the electrodes serves as both the electrolyte and the reactant for chlorine evolution. During the course of electrolysis, the Cl.sub.2 and H.sub.2 gases formed at the two electrodes cause a "blinding effect" by forming a gaseous film on the electrode surface, thereby further impeding the ionic pathway between the electrodes and the aqueous HCl. Use of an aqueous electrolyte coupled with a non-conductive polymeric membrane as a separator results in additional resistance and therefore reduces electrical efficiency of the system.
Controlling and minimizing oxygen evolution by the side reaction are important considerations in Balko, U.S. Pat. No. 4,311,568. Balko describes a process for aqueous HCl electrolysis which uses an electrolytic cell having a solid polymer electrolyte membrane. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of cell components. The design and configuration of the anode pore size and electrode thickness maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from FIGS. 3 to 5 of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko, can run at higher current densities, but is limited by the deleterious effects of oxygen evolution especially on carbon-containing electrodes. Further, while Balko teaches thinner anodes provide better performance, anodes having thickness less than 6 microns demonstrated unacceptable performance.
The electrochemical cell of Balko, U.S. Pat. No. 4,311,568, employs a membrane and electrodes that are physically separate elements, which have been bonded together using high pressure. Such an arrangement has non-uniformities in both the membrane and the electrodes, resulting in uneven contact therebetween and less utilization of the catalyst than if the contact between the membrane and the electrodes were uniform. Accordingly, the current density of such a cell is limited by catalyst utilization.
Faita, EPO 785 294 A1, describes a process for aqueous HCl electrolysis in an electrochemical cell, wherein the anode and cathode are separated by an ion exchange membrane and are constructed from titanium or titanium alloys, which are less costly than conventional graphite-based materials. The process is further characterized by addition of an oxidizing compound to the aqueous HCl solution, typically ferric ion, and feeding an oxygen-containing gas to the cathode to generate water, to maintain titanium in a passive condition. However, Faita teaches only low current densities of 3-4 kA/m.sup.2, is limited to an oxygen reducing cathode and further, does not address low acid concentration and associated problems relating to generating oxygen at the anode.
Uehara, et al., in a series of articles (Denki Kagaku oyobi Kogyo Butsuri Kagaku, 1990, vol. 58, no. 4, pp. 360-7; Denki Kagaku oyobi Butsuri Kagaku, 1990, vol. 58, no. 5, pp. 459-65; Denki Kagaku, 1990, vol. 58, no. 11, pp. 1052-8; and Osaka Kogyo Gijutsu Shikensho Kiho, 1993, vol. 44, no. 2, pp. 47-52) disclose studies on aqueous HCl electrolysis using a solid polymer electrolyte membrane. Electrodes were prepared using chemical plating methods. Voltage characteristics were studied under conditions, wherein the anode and cathode HCl concentrations were equal. Current efficiency was studied in terms of hydrogen and chlorine yields, with and without an external supply to feed the cathode. Differences between having the electrocatalyst bonded and non-bonded to the membrane were also disclosed.
While processes for the electrolysis of aqueous solutions of HCl are known, it is still desirable to improve upon these processes to make them more attractive economically as a means to recycle byproduct hydrochloric acid solutions. It would be desirable to have a process for the electrolysis of aqueous HCl to generate chlorine having either separately, or in combination, improvements in current density, cell voltage and lower oxygen evolution, particularly at low HCl concentration. Current density is related to reaction rate. Higher current densities provide higher reaction rates, allowing for smaller reactors, and therefore lower investment. Cell voltage is related to energy requirements for the process. Lower cell voltage requires less energy and therefore lower operating costs. Lower oxygen evolution reduces current efficiency losses and corrosion of carbon-based cell components. Operating with a lower outlet HCl concentration allows higher per pass conversion of the HCl feed, especially when the HCl feed concentration is limited by the HCl-water azeotrope (ca. 20 wt % HCl). The present invention provides such a process for the electrolysis of aqueous solutions of HCl.